Some socket options

In fact, both network socket client & server will call a set_common_sockopts function which will set some socket options according to command line options: 

  1. void
  2. set_common_sockopts(int s, int af)
  3. {
  4.     int x = 1;
  6.     if (Sflag) {
  7.         if (setsockopt(s, IPPROTO_TCP, TCP_MD5SIG,
  8.             &x, sizeof(x)) == -1)
  9.             err(1, NULL);
  10.     }
  11.     if (Dflag) {
  12.         if (setsockopt(s, SOL_SOCKET, SO_DEBUG,
  13.             &x, sizeof(x)) == -1)
  14.             err(1, NULL);
  15.     }
  16.     if (Tflag != -1) {
  17.         if (af == AF_INET && setsockopt(s, IPPROTO_IP,
  18.             IP_TOS, &Tflag, sizeof(Tflag)) == -1)
  19.             err(1, "set IP ToS");
  21.         else if (af == AF_INET6 && setsockopt(s, IPPROTO_IPV6,
  22.             IPV6_TCLASS, &Tflag, sizeof(Tflag)) == -1)
  23.             err(1, "set IPv6 traffic class");
  24.     }
  25.     if (Iflag) {
  26.         if (setsockopt(s, SOL_SOCKET, SO_RCVBUF,
  27.             &Iflag, sizeof(Iflag)) == -1)
  28.             err(1, "set TCP receive buffer size");
  29.     }
  30.     if (Oflag) {
  31.         if (setsockopt(s, SOL_SOCKET, SO_SNDBUF,
  32.             &Oflag, sizeof(Oflag)) == -1)
  33.             err(1, "set TCP send buffer size");
  34.     }
  36.     if (ttl != -1) {
  37.         if (af == AF_INET && setsockopt(s, IPPROTO_IP,
  38.             IP_TTL, &ttl, sizeof(ttl)))
  39.             err(1, "set IP TTL");
  41.         else if (af == AF_INET6 && setsockopt(s, IPPROTO_IPV6,
  42.             IPV6_UNICAST_HOPS, &ttl, sizeof(ttl)))
  43.             err(1, "set IPv6 unicast hops");
  44.     }
  46.     if (minttl != -1) {
  47.         if (af == AF_INET && setsockopt(s, IPPROTO_IP,
  48.             IP_MINTTL, &minttl, sizeof(minttl)))
  49.             err(1, "set IP min TTL");
  51.         else if (af == AF_INET6 && setsockopt(s, IPPROTO_IPV6,
  52.             IPV6_MINHOPCOUNT, &minttl, sizeof(minttl)))
  53.             err(1, "set IPv6 min hop count");
  54.     }
  55. }

Below is the table which summarize these options:










VariableCommand optionSocket level & optionMeaning
Sflag-SIPPROTO_TCP&TCP_MD5SIGUses TCP MD5 signatures per RFC 2385
Dflag-DSOL_SOCKET&SO_DEBUGEnables recording of debugging information
Tflag-TIPPROTO_IP&IP_TOS IPPROTO_IPV6&IPV6_TCLASSChanges IPv4 TOS value (refer process_tos_opt function) or sets IPv6 traffic class field
Iflag-I lengthSOL_SOCKET&SO_RCVBUFSets buffer size for input
Oflag -O lengthSOL_SOCKET&SO_SNDBUFSets buffer size for output
ttl -M ttlIPPROTO_IP&IP_TTL IPPROTO_IPV6&IPV6_UNICAST_HOPSSets the TTL / hop limit of outgoing packets
minttl -m minttlIPPROTO_IP&IP_MINTTL IPPROTO_IPV6&IPV6_MINHOPCOUNTDiscards packets with a TTL lower than the option value